Instead of astronauts going to the rock, the rock will come to them.

NASA’s ambitious new plan to capture an asteroid and bring it to the Moon may seem exotic and overly complex, but it arose from relatively mundane scientific considerations and percolated among space scientists for more than two years before Administrator Charles Bolden announced it Wednesday.

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NASA's plan to retrieve an asteroid starts with a robotic mission to rendezvous with a 25-foot space rock in 2019 and place it in a capture bag. The asteroid would then be returned to the neighborhood of Earth and the Moon for closer examination by astronauts, who would launch in 2021 in the new Orion spacecraft.
Video: NASA

It will not be simple. Planners must find a candidate asteroid much smaller than those routinely detected by ground-based telescopes. Then, in 2019, a robotic spacecraft using advanced solar electric propulsion will have to travel to the asteroid and capture it, probably using an inflatable device like a large fabric bag.

Finally, the spacecraft must carry the asteroid to the vicinity of the Moon and decide whether to place it in lunar orbit or park it on the lunar far side where the combined gravity of Earth and the Moon will hold it in place. Only then would astronauts pay the asteroid a visit. NASA has budgeted $105 million this year to fund initial work on the project. NASA officials say they can do the whole job for $2.5 billion.

Asteroid capture became an attractive option for NASA in 2010 after President Obama decided to scrap plans for astronauts to return to the Moon and announced that human space flight would instead focus on sending astronauts to an asteroid by 2025.

The dilemma for mission planners was that candidate asteroids were scarce, and sending astronauts to an asteroid in many respects would be just as difficult as sending them to Mars. So, reasoned some scientists, instead of sending astronauts to an asteroid, why not bring the asteroid to the astronauts?

“You get into it, and it becomes obvious that this is the only way you could get astronauts to an asteroid in the 2020s,” said Louis Friedman, executive director emeritus of the Pasadena, California-based Planetary Society. “The crazy part is going out and getting an asteroid, so you put that part in the robotic world. Then you open the mission up to the astronauts.”

The Keck study, the most comprehensive treatment of the capture plan, was published last year. It determined that the optimum-sized asteroid that could be captured and delivered to the vicinity of the Moon by 2025 would be around 7 meters (23 feet) in diameter with a mass of 500 tons. NASA briefers this week agreed with the mass, but increased the diameter to 10 meters (33 feet) and suggested the asteroid could reach the Moon as early as 2021.

The target asteroid would be a “near-Earth object” with an orbit that brings it within 4.6 million miles of Earth. The rock would be “smaller than what we look at normally,” says deputy NASA administrator Robert Lightfoot, one reason NASA is doubling the size of its asteroid detection budget to $40 million next year. Telescopes would detect candidates, track their orbits and determine their composition.

To reach the asteroid, capture it and bring it to the Moon, NASA wants to use a robotic spacecraft equipped with low-thrust engines to travel from Low Earth Orbit to the candidate asteroid. The NASA budget allots $45 million next year to accelerate development of high-powered solar electric propulsion.

The Keck study proposes that once the spacecraft reaches the asteroid, it would deploy a cylindrical fabric bag, move close to the asteroid, match its spin and gently capture it (see animation here). Then it will de-spin the asteroid and take it to the Moon.Lightfoot told reporters this week that NASA’s OSIRIS-REx mission to collect and return a sample of asteroid 1999RQ36, scheduled for launch in 2016, will serve as an important precursor.

With the asteroid in the bag and on its way to the Moon, planners must decide where to put it and how to organize a human spaceflight mission to rendezvous with it, study it and examine its potential for usable resources like water and rare earth minerals. The NASA budget has allotted $40 million next year to study this phase of the mission.

NASA briefers said they would use the agency’s new Orion spacecraft, currently under development, as the basic vehicle. Although the agency did not specify how it intended to accomplish the rendezvous, planners have acknowledged that they are considering at least two options.

The first, and simplest, would put the asteroid in lunar orbit and have Orion rendezvous with it there. If something went wrong and the asteroid destabilized, it would simply plunge to the surface of the Moon.

A more ambitious possibility is NASA’s so-called “Gateway” project to put Orion in orbit around Lagrange 2, a spot on the far side of the Moon where the gravitational pull of the Moon and the Earth cancel each other out. Orion would need far less fuel for station-keeping at L2 than in lunar orbit.

L2’s disadvantage is that objects there have a tendency to drift, and the asteroid could fall to Earth, although Friedman says it would most likely burn up in the atmosphere. NASA has been studying Gateway for almost two years, but has not yet made a decision how far to take the idea.